Sperm competition is a form of post-copulatory sexual selection whereby male sperm simultaneously physically compete to fertilize a single ovum. Sperm competition occurs between sperm from two or more rival males when they make an attempt to fertilize a female within a sufficiently short period of time. This results primarily as a consequence of polyandrous mating systems, or due to extra-pair copulations of females, which increases the chance of cuckoldry, in which the male mate raises a child that is not genetically related to him. Sperm competition among males has resulted in numerous physiological and psychological adaptations, including the relative size of testes, the size of the sperm midpiece, prudent sperm allocation, and behaviors relating to sexual coercion, however this is not without consequences: the production of large amounts of sperm is costly and therefore, researchers have predicted that males will produce larger amounts of semen when there is a perceived or known increase in sperm competition risk. Sperm competition is not exclusive to humans, and has been studied extensively in other primates, as well as throughout much of the animal kingdom. The differing rates of sperm competition among other primates indicates that sperm competition is highest in primates with multi-male breeding systems, and lowest in primates with single-male breeding systems. Compared to other animals, and primates in particular, humans show low-to-intermediate levels of sperm competition, suggesting that humans have a history of little selection pressure for sperm competition.
Physiological adaptations to sperm competition Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in their evolutionary history. Nevertheless, there is a large body of research that explores the physiological adaptations males do have for sperm competition.
Testis size and body weight Evidence suggests that, among the great apes, relative testis size is associated with the breeding system of each primate species. In humans, testis size relative to body weight is intermediate between monogamous primates (such as gorillas) and promiscuous primates (such as chimpanzees), indicating an evolutionary history of moderate selection pressures for sperm competition.
Ejaculate volume The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males' testes, ejaculate volume is also intermediate between primates with high and low levels of sperm competition. Human males, like other animals, exhibit prudent sperm allocation, a physiological response to the high cost of sperm production as it relates to the actual or perceived risk of sperm competition at each insemination. In situations where the risk of sperm competition is higher, males will allocate more energy to producing higher ejaculate volumes. Studies have found that the volume of sperm does vary between ejaculates, and that sperm produced during copulatory ejaculations are of a higher quality (younger, more motile, etc.) than those sperm produced during masturbatory ejaculates or nocturnal emissions. This suggests that, at least within males, there is evidence of allocation of higher quality sperm production for copulatory purposes. Researchers have suggested that males produce more and higher quality sperm after spending time apart from their partners, implying that males are responding to an increased risk of sperm competition, although this view has been challenged in recent years. It is also possible that males may be producing larger volumes of sperm in response to actions from their partners, or it may be that males who produce larger volumes of sperm may be more likely to spend more time away from their partners.
Size of sperm midpiece
The size of the sperm midpiece is determined in part by the volume of mitochondria in the sperm. Sperm midpiece size is tied to sperm competition in that individuals with a larger midpiece will have more mitochondria, and will thus have more highly motile sperm than those with a lower volume of mitochondria. Among humans, as with relative testis size and ejaculate volume, the size of the sperm midpiece is small compared to other primates, and is most similar in size to that of primates with low levels of sperm competition, supporting the theory that humans have had an evolutionary history of intermediate levels of sperm competition.
Penis anatomy
Several features of the anatomy of the human penis are proposed to serve as adaptations to sperm competition, including the length of the penis and the shape of the penile head. By weight, the relative penis size of human males is similar to that of chimpanzees, although the overall length of the human penis is the largest among primates. It has been suggested by some authors that penis size is constrained by the size of the female reproductive tract (which, in turn is likely constrained by the availability of space in the female body), and that longer penises may have an advantage in depositing semen closer to the female cervix. Other studies have suggested that over our evolutionary history, the penis would have been conspicuous without clothing, and may have evolved its increased size due to female preference for longer penises. The shape of the glans and coronal ridge of the penis may function to displace semen from rival males, although displacement of semen is only observed when the penis is inserted a minimum of 75% of its length into the vagina. After allegations of female infidelity or separation from their partner, both men and women report that men thrusted the penis more deeply and more quickly into the vagina at the couple's next copulation.
Psychological adaptations to sperm competition In addition to physiological adaptations to sperm competition, men also have been shown to have psychological adaptations, including certain copulatory behaviors, behaviors relating to sexual coercion, investment in relationships, sexual arousal, performance of oral sex, and mate choice.
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